(USA TODAY) - An Oklahoma death row inmate died of a heart attack Tuesday night more than 40 minutes after his execution was halted because the lethal injection of three drugs was botched.

The execution of a second convicted murderer was postponed.

Corrections Director Robert Patton halted the execution of Clayton Lockett about 20 minutes after the first drug was administered, the Associated Press reported. He said there was a vein failure that prevented the deadly chemicals from reaching Lockett.

Witnesses said Lockett writhed and convulsed on the gurney, shaking uncontrollably.

Patton said he died after receiving all three drugs.

"There was some concern at that time that the drugs were not having that (desired) effect, and the doctor observed the line at that time and determined the line had blown," Patton said at a news conference afterward, referring to Lockett's vein rupturing.

The 38-year-old Lockett, who was convicted of shooting 19-year-old Stephanie Neiman and watching two accomplices bury her alive in 1999, was pronounced dead in the execution chamber at 7:06 p.m. CT.

The execution began at 6:23 p.m., and he was unconscious 10 minutes later. His death came 43 minutes after the drugs began flowing.

Here's how The Oklahoman described the scene at the death house in McAlester:

Lockett grimaced and tensed his body several times over a three minute period before the execution was shielded from the press. After being declared unconscious ten minutes into the process, Lockett spoke at three separate moments. The first two were inaudible, however the third time he spoke, Lockett said the word "man."The paper said officials closed the curtains 16 minutes into the execution and ended it 20 minutes later.

"It was extremely difficult to watch," Lockett's attorney, David Autry, said afterward.

Charles Warner's execution had been set for 8 p.m. Tuesday. The 46-year-old was convicted of raping and killing his roommate's 11-month-old daughter in 1997. He has maintained his innocence.

It was the first time since 1937 that two men were to have been executed on the same day in Oklahoma, although it has happened in other states since the death penalty was reinstated in the U.S. in 1976. The last double execution was in Texas in 2000.

Lockett and Warner had sued the state for refusing to disclose details about the execution drugs, including where Oklahoma obtained them.

An Oklahoma death row inmate died of a heart attack Tuesday night more than 40 minutes after his execution was halted because the lethal injection of three drugs was botched. Now death penalty opponents are demanding change.

The case, filed as a civil matter, placed Oklahoma's two highest courts at odds and prompted calls for the impeachment of state Supreme Court justices after the court last week issued a rare stay of execution. The high court later dissolved its stay and dismissed the inmates' claim that they were entitled to know the source of the drugs.

By then, Gov. Mary Fallin had weighed in on the matter by issuing a stay of execution of her own — a one-week delay in Lockett's execution that resulted in both men being scheduled to die on the same day.

"Our goal is to make sure justice is served," Fallin said Tuesday. "The courts have ruled, and there is no doubt as to the guilt of the perpetrators of the crimes."

Lockett's request of steak, shrimp, a large baked potato and a Kentucky bourbon pecan pie was denied because it exceeded the $15 limit, and he declined a separate offer from the warden for a dinner from Western Sizzlin, prison officials said.